Plasma etch processes are employed in microelectronic circuit fabrication to define thin film structures on semiconductor wafers or workpieces. Generally, a disc-shaped workpiece is processed in a cylindrical reactor chamber. Features sizes in the thin film structures formed by the etch process can be as small as tens of nanometers, for example. Uniformity of etch rate distribution across the entire surface of the workpiece is critical in attaining workable devices. The etch rate distribution reflects the plasma ion density distribution across the workpiece surface existing during the plasma etch processing of the workpiece. Etch processes can employ an inductively coupled RF plasma in which the plasma source consists of a coil antenna over the ceiling of the reactor chamber. The etch rate distribution can have a radial non-uniformity, in which the non-uniformity pattern is generally symmetrical about the cylindrical axis of symmetry of the reactor chamber. For example, the etch rate distribution may reflect a plasma ion density distribution that is, predominantly, either center-high or center low. Such a radial pattern of non-uniformity can be corrected by dividing the ceiling coil antenna into two or more concentric coil antennas that are separate from one another and are independently powered with RF power. Radial non-uniformity in etch is corrected in such a reactor by adjusting the RF power levels independently delivered to the separate concentric coil antennas. While this arrangement works well in correcting radial non-uniformities in etch rate distribution, it is not well-suited for correcting for asymmetrical non-uniformities in etch rate distribution. Such asymmetrical non-uniformities may be referred to as “skew” non-uniformities, and typically are manifested as a difference between etch rates on opposite sides of the workpiece. As one simplified example, one half of the workpiece may experience a higher etch rate than the other half. Under real production conditions, it is often found that the etch rate distribution measured across the surface of the workpiece has both radial non-uniformity and skew non-uniformity in combination. If the skew non-uniformity could be somehow corrected or eliminated, then the remaining non-uniformity, namely the radial non-uniformity, could be eliminated by apportioning the RF power levels delivered to the different concentric overhead coil antennas. The result would be correction of all etch rate distribution non-uniformity across the workpiece surface. The problem is how to eliminate the skew non-uniformity in etch rate distribution.